A lot of good suggestions. Logs, railroad ties, tires, dirt, etc.. They all work great. I'll tell you what NOT to do. When my son was on H.S. shop class I got some good 1/2" steel plate and had him weld up a nice bullet trap. I figured that would be super safe for shooting in the yard. Well it would take anything from .22 to .45 no problem. Except it made one hell of a lot of noise. The clang of the bullet hitting the 45 degree steel back plate sounded like ricochets to one of the neighbors. Being the cool dude that he is, he came over to investigate instead of calling the cops. I assurred him that no bullets were leaving the trap (which I had in a 6' deep gulley for double protection) but immediately gave up on using the trap. Now we just shoot into pieces of log or timber with a dirt berm backstop.